Environmental Engineering Reference
In-Depth Information
In folded sequences of sandstones with interbeds of siltstones or shales, landslides are
relatively common where dipping beds daylight on steep slopes. Slides are also common on
dipslopes. Examples of these types of sliding involving sandstones are given in Chapter 2,
Section 2.10.3.4 and 2.10.3.5.
3.6.5
Sandstones and similar rocks - list of questions
- Relatively high porosity, permeable?
- Gypsum or anhydrite present as cement?
- Quartzites: High quarrying and handling costs, difficult to compact?
- Rocks of medium or lower strength may not produce free-draining rockfill?
- Interbeds of shale or claystone?
- Bedding-surface faults at bed boundaries?
- Horizontal beds: Open joints and bedding surface crushed seams near surface due to
stress relief?
- Horizontal beds with shale interbeds: Cambering and collapse due to removal of sup-
port by weathering shale?
- Landsliding in colluvium developed on weathering sandstone/shale slopes?
3.7
CARBONATE ROCKS
Carbonate rocks are defined here as those which contain significant amounts of the solu-
ble minerals calcite, aragonite or dolomite in their substance fabrics. The most common
are sedimentary carbonate rocks, marble (metamorphosed carbonate rock), and calc-sili-
cate rocks formed by the metamorphism of impure carbonate rocks.
It is necessary to consider carbonate rocks in two categories, based on the dominant
carbonate minerals present in each category.
Geologically young carbonate rocks (Category Y)
.
Category Y
carbonate rocks are usu-
ally of Tertiary, or younger, age. Most comprise loosely packed, weakly cemented shell
fragments and are porous and weak to very weak. Rarely they can be well cemented and
dense. The carbonate minerals are mostly aragonite and high-magnesian calcite
(Friedman 1964, 1975; Molenaar and Venmans, 1993; Prothero and Schwab, 1996).
These two minerals formed all original marine carbonate sediments and are forming in
present day deposits. There is field and laboratory evidence (see
Sections 3.7.1.3
and
3.7.7.2
) that high-magnesian calcite is more susceptible to dissolution and cementation
than aragonite and calcite. With time, exposure to fresh water, compaction and recrys-
tallisation both aragonite and high-magnesian calcite eventually revert to calcite, and the
rock becomes
Category O
.
Geologically old carbonate rocks (Category O). Category O
carbonate rocks are gen-
erally of Mesozoic or older age and are usually dense, non porous and range from strong
to extremely strong. They are formed by the minerals calcite or dolomite. They include
marble, which comprises coarsely crystalline calcite and is usually dense, non porous and
strong to very strong. Also included are calc-silicate rocks, which contain carbonate min-
erals together with silicate minerals often including olivine, diopside and garnet.
The carbonate rocks at the sites of most large dams are
Category O
. The exceptions
mentioned in this topic are Kopili Dam, Perdikas, Montejagne and May Dams, each of
which failed to store water (
Table 3.4
).
Table 3.2
proposed by Dearman (1981) is a practical engineering classification for sed-
imentary carbonate rocks. Dearman (1981), Cruden and Hu (1988) and Bell (1981,
1983b and 1992) present data on physical properties of some carbonate rocks. Fookes
and Higginbottom (1975) provide a table which classifies and names various types of
